1. Field of the Invention
The invention relates to a method for the transmission of data in a communication system, in which a transmittal data stream with serially successive data is transmitted via a time-variant transmission channel.
2. Description of the Related Art
If on the basis of the receiving-end checking data information, an errored data block was detected, the corresponding data block will be rejected in the case of the ARQ method and requested again at the transmission end.
With a hybrid ARQ method, the errored data block that was first transmitted is buffered and re-requested at the transmission end. The re-requested data block and the buffered data block are combined with one another in the receiver. The resulting data block is then checked again for errors using the checking procedure.
With the hybrid ARQ method the data block is re-transmitted in accordance with the pure ARQ method: The data block is re-transmitted unchanged and coded in the same way. A method known as “chase combining” is used as the combination method.
As an alternative, the block can be transmitted in accordance with the “incremental redundancy” method. Here the coding of the data block to be re-transmitted is changed. As a result, an additional redundancy method for error correction is made available at the receiver. The buffered data block and the re-transmitted data block are combined by a so-called “code combining” method.
In the case of radio communication systems, it is necessary to adapt a data transmission rate with a high data throughput optimally and dynamically to the properties of a radio transmission channel which, in general, change over time (time-variant) through statistical fluctuations and interferences within the radio transmission channel. However, in the case of an increased data transmission rate, the susceptibility of data transmission to errors also increases because the capacity limit of the radio transmission channel is being approached.
The problems resulting from time-variant radio transmission channels are, for example, well-known from the mobile radio standards GSM, UMTS, HiperLAN, etc. and are particularly marked for strong power fluctuations in a receive signal as well as by the noise that overlaps and interferes with this received signal caused by a receiving amplifier arranged at the receiving end.
The power fluctuations in this case depend on the location and movement of a mobile user.
Telephone lines for ADSL connections, cables for cable television and optical fiber cables can also be seen as time-variant transmission channels.
Data blocks to be transmitted are protected against transmission errors by redundancy methods, forwards error correction methods or by a special error detection coding method. As a result, in each case a proportion of useful data is reduced accordingly within a data block to be transmitted.
Therefore, while in the case of a transmission channel with poor transmission properties a large amount of redundancy data is required in a data block, it is known that for transmission channels with good transmission properties (with a typical bit error rate BER <10-5), a checksum for error detection is already sufficient, in which case a maximum proportion of useful data is obtained in this instance.
In the case of the hybrid ARQ method, by using the combination method, a receiving-end signal-to-noise ratio SNR is improved to such an extent that an error free reception is made possible. However, a disadvantage of the hybrid ARQ method can be seen in the fact that, by repeatedly transmitting entire data blocks, only a rough grading of the data rate and, as a result, only a rough adaptation to the properties of the radio transmission channel is made possible. large memory capacities must be provided at the receiving end to buffer errored data blocks. Re-requesting and transmitting errored data blocks leads to delays in the data stream or an effective useful data throughput is reduced.
For data transmissions in a communication system, especially in a radio communication system, so-called automatic repeat request transmission methods (ARQ transmission methods) or hybrid ARQ transmission methods are known in which an input data stream with serially successive data is transmitted by radio and divided into data blocks. In this case, each individual data block to be transmitted has a test data sequence added as a prefix which at the receiving end allows information to be provided about whether or not a data block was transmitted without errors. This test data sequence can be embodied, for example, as a checksum covering the data block or as a CRC data sequence for a cyclic redundancy check.